The basic techniques of arthroscopic surgery are thoroughly reviewed in medical textbooks such as O'Connor's Textbook of Arthroscopic Surgery, Heshmat Shahriaree--Editor, J. B. Lippincott Company (1984) [hereinafter referred to as O'Conner's]; and Arthroscopic Surgery Principles & Practice, Lanny L. Johnson, The C. V. Mosby Company (1986).
As is discussed in these references, arthroscopic surgery involves the insertion of a device, known as an arthroscope, into a joint region, such as the knee, elbow or shoulder, in order to allow the surgeon to view the internal condition of the joint.
In arthroscopic knee surgery, it is common practice to fill the suprapatellar pouch that surrounds the knee with a clear liquid, such as saline irrigation solution, in order to keep the field of view through the arthroscope clear. Since even a small amount of blood will cause the clear solution to become cloudy, means for draining and replenishing the irrigation solution are needed throughout the duration of the surgery.
This is typically accomplished by inserting a rigid inflow cannula into the suprapatellar pouch, above the patella, and attaching a tube between the inflow cannula and an elevated fluid reservoir. The arthroscope is inserted into the knee and a gravity or suction device is attached to one port of the arthroscope.
Fluid from the fluid reservoir is introduced into the suprapatellar pouch by opening an "on-off" valve on the inflow cannula. Fluid then enters the suprapatellar pouch under the force of gravity or mechanically induced pressure and is removed from the suprapatellar pouch by activating the suction or free flow gravity device attached to the arthroscope.
Alternatively, fluid can be introduced through the arthroscope and removed with a gravity device attached to the cannula (e.g. a flexible plastic tube running from the cannula to a bucket on the floor). In this method, the cannula is referred to as an outflow (or egress) cannula or as a drainage needle.
Typically, the cannulae that are used in arthroscopic knee surgery are rigid (unbending) cannulae. An example of a such a cannula is the rigid metal cannula shown in O'Conner's at page 29 (called a "drainage needle" because of its use in the second method discussed above). An example of a teflon outflow cannula is shown in O'Conner's at page 172.
During arthroscopic knee surgery, the surgeon typically rotates and bends the patient's knee, while the arthroscope and inflow/outflow cannula are positioned in the knee, in order to obtain a different view of the joint structure under examination.
This rotation and bending of the knee during the arthroscopic surgery can cause numerous problems with the rigid cannula. First, when the knee is bent, the flow of fluid through the rigid cannula is frequently completely blocked by the synovial lining of the suprapatellar pouch closing down on the end of the cannula. Second, when the knee is bent or rotated, a rigid cannula can bruise or puncture the suprapatellar pouch or other tissue.